Acoustic logging systems are routinely used in the oil and gas industry to measure formation acoustic properties of earth formation penetrated by a well borehole. These properties include the compressional and shear velocities of the formation, which are subsequently used to determine a variety of formation parameters of interest including, but not limited to; porosity, lithology, density and pore pressure. Additionally, acoustic logging systems are used to produce acoustic images of the borehole from which well conditions and other geological features can be investigated. Other applications of acoustic logging measurements include seismic correlation and rock mechanic determination.
The above mentioned acoustic measurements typically need to be corrected for any formation anisotrophic effects before parameters of interest can be determined from the measured parameters. Furthermore, anisotrophy parameters are themselves parameters of interest, and are used in a variety of geophysical applications including cross-well seismic measurements, convention seismic interpretations, and the like. Elastic anisotropy manifests itself as the directional dependence of sound speed in earth formation. Anisotropy in earth formation may be due to intrinsic microstructure, such as the case in shales, or may be due to mesostructure, such as fractures, or may be due to macrostructure such as layering due to sedimentation. Whatever the cause for anisotropy may be, good estimates of elastic properties of anisotropic media are required in resolving seismic images accurately, in interpreting borehole logs and in estimating drilling mechanics parameters.
Most formations have anisotropic structure resulting from layering, micro fractures, or orientation of mineral deposits in a certain direction. This internal stress causes the shear velocity to vary with propagation direction. Transverse isotropy (TI) is commonly used to model formation anisotropy. One example of TI anisotropy is horizontal transverse isotropy (HTI) where the axis of anisotropic symmetry is horizontal. Another example of TI anisotropy is vertical transverse isotropy, where the axis of anisotropic symmetry is vertical. Specific examples of these TI anisotropy formations are vertical fracturing along the borehole axis and horizontal bedding planes in a vertical well.
Formation anisotropy can be determined with acoustic logging-while-drilling (LWD) or measurement-while-drilling (MWD) systems. Formation anisotropy can also be determined with acoustic wireline systems after the borehole drilling operation is complete. MWD, LWD, and wireline acoustic logging systems comprising monopole and dipole acoustic sources have been used in the prior art as shown, for example, in U.S. Pat. Nos. 7,623,412 B2, 5,808,963, 6,714,480 B2, 7,310,285 B2, 7,646,674 B2, which are incorporated herein by reference. There are operational and environment factors that limit practical source frequencies, especially in MWD and LWD systems. This topic will be discussed in detail in subsequent sections of this disclosure.